The present invention relates generally to a two-way CATV system and, more particularly, to a two-way CATV system capable of restraining upstream ingress noises which might become a problem in the case of performing communication services by utilizing a CATV oriented transmission path (CATV line), and of facilitating the maintenance.
A CATV (Community Antenna Television) system is constructed for the purpose of retransmitting broadcast programs in TV broadcasting, satellite broadcasting, local origination broadcasting, etc., wherein information signals are transmitted only in a downstream direction to individual subscriber homes, etc. from a CATV center station (head end).
Over the recent years, a two-way CATV system has been introduced, wherein not only a broadcasting device but also a two-way communication device are installed within a CATV center station of the CATV system, the CATV transmission being structured as an optical coaxial hybrid transmission path composed of both an optical fiber cable and a coaxial cable, and amplifiers for relay amplifications being disposed in order to compensate for a loss of the transmission path in broadband. The two-way CATV system is capable of transmitting upstream signals by utilizing a frequency band unused for transmitting downstream signals for a broadcast program, and services to promote a fusion of broadcasting services and communications services.
In the CATV system of which a main purpose is to provide the broadcasting services, a transmission quality of the downstream signals (a transmission band is, e.g., 70 MHz through 750 MHz) is managed. In the two-way CATV system introduced so far, however, with respect to upstream signals (a transmission band is, e.g., 10 MHz to 55 MHz), the usage thereof is limited to a data transmission of monitor signals of home terminals of subscriber homes, amplifiers on the CATV transmission path and power supply devices to the amplifiers, and to temporary utilization for relay transmission to a CATV center station from an event site, and hence a transmission quality thereof is not necessarily sufficiently managed at a high quality level with stability.
In the case of utilizing the CATV transmission path for communications, it is of much importance to manage the upstream signals (up-signals) together with the downstream signals (down-signals). In particular, an important factor for determining a quality of the two-way CATV system may be so-called ingress noises, in which the noises caused from within a multiplicity of amplifiers, tap-off lines and the subscriber homes, enter in the transmission band of the upstream signals.
The ingress noises are, it is generally considered, induced by the following causes, and counter measures against those noises are now in the process of being taken. The first cause is the thermal noises generated from the amplifiers and the terminals in the subscriber homes. A countermeasure against first cause is that the thermal noises are inevitably produced and therefore theoretically calculated, and the numbers of the amplifiers and of the terminals are limited so that no problems in terms of the system might arise. Further, the second cause may be jump-in noises from outside, viz., the external noises derived from amateur radio operations and citizen's band operations. A countermeasure against the second cause is that the system is designed and maintained to ensure over a predetermined level of immunity against the external noises between a trunk and branch lines of the transmission path and in-form wiring. Further, a fault of the device and fluctuations in adjustment thereof might cause otherwise the ingress noises, however, a steady noise occurs, and the maintenance is comparatively easy to perform.
Incidentally, the coaxial connector of the coaxial cable which is a passive part, has hitherto been conceived as a part of the linear CATV transmission path, however, it was confirmed from results of measurement by a certain CATV operator and in a common path distortion described in the technological report “Broadband Return Systems for Hybrid Fiber/Coax Cable TV Networks” by Donald Raskin, Dean Stoneback, published in December, 1997, Pranntice Hall PTR” that the electric contact surface of the coaxial connector, i.e., an easy-to-slacken portion by contacts of different kinds of base metals (Cu, Al, etc.) operates non-linearly due to a diode effect, and hence inter-modulation distortion noises of downstream RF carriers occur also in the transmission band of the upstream signals.
The upstream ingress noises attributed to the inter-modulation distortion by the non-linear characteristic of the electric contact surface of the coaxial connector, have the following distinctive aspects. First, a multiplicity of downstream signals are mixed with a complicity, and therefore the ingress noises are produced over the entire transmission bands of the upstream signals. However, a periodic spectrum at an interval of 6 MHz or 2 MHz is observed because of receiving an influence of a carrier array spectrum of a downstream signal channel. Second, as for a frequency of the occurrence thereof, there is a small possibility in which the noises might occur steadily, and there might be some cases where the occurrence of noises continues occasionally for several seconds to several minutes and continues for several hours.
Third, the very place where the noises occur is the coaxial connector through which the high-level downstream signals just under the output of an extension amplifier, and the noises, an occurred state thereof being unstable, might disappear during rework. If incompletely reworked, however, there is a possibility in which the noises again occur. Fourth, the upstream ingress noises due to the inter-modulation distortion exhibits a periodic spectrum of which a peak has a magnitude larger by 10 dB trough 20 dB than the thermal noises produced mainly from the amplifier.